Signal Isolation Techniques- analog and digital signal isolation

There are many applications where it is required for one system to have no direct electrical connection with the other system. Such isolation is called Galvanic isolation. This is necessary to avoid the possibility of dangerous voltages or currents from one half of the system causing damage to the other, or to break ground loop. Such a system is said to be "isolated", and the arrangement that passes a signal without galvanic connections is known as an isolation barrier.

The protection of an isolation barrier works in both directions. The common applications where a sensor may accidentally encounter high voltages and the system it is driving must be protected.

An isolation amplifier provides dc isolation between input and output. It is used for the protection of human life or sensitive equipment in those applications where high-voltage transients or power leakage are possible. An isolation amplifier consists of two electrically isolated stages. The input stage and the output stage are separated from each other by an isolation barrier so that a signal must be processed in order to be coupled across the isolation barrier.

Complete isolation of two electrical system include- signal isolation, power isolation.

Application

·         To protect human operators,

·         To protect low-voltage circuitry from high voltages,

·         To improve noise immunity,

·         To reject common mode voltage,

·         To eliminate ground loop,

·         To handle ground potential differences between communicating subsystems.

Types of signal isolation

1.      Optical Isolation

2.      Transformer/ magnetic/ inductive isolation

3.      Capacitive isolation

Some isolation amplifiers use optical coupling or transformer coupling to provide isolation between the stages. However, many modern isolation amplifiers use capacitive coupling for isolation. Each stage has separate supply voltages and grounds so that there are no common electrical paths between them.

The most common isolation amplifiers use transformers, which exploit magnetic fields, and another common type uses small high voltage capacitors, exploiting electric fields.

Optical isolation

Optoisolators, which consist of an LED and a photodiode/ phototransistor, provide isolation by using light. Optical isolators are fast and cheap, and can be made with very high voltage ratings (4 -7 kV is one of the more common ratings), but they have poor analog linearity, and are not usually suitable for direct coupling of precision analog signals.

Analog optocoupler

The analog optocouplers can be used to isolate analog signals in a wide variety of applications that require good stability, linearity, bandwidth and low cost. HCNR200/201 is popular analog optocoupler. Their circuit schematic is as shown below.

 

The figure below shows  the working circuit of analog voltage isolator using HCNR200.

The ratio of R3/R1 can be selected to provide amplification to the input signal. The ground for input stage (left side) must be separated from ground for output stage (right side). Separate set of power supplies or isolated DC-to-DC converters can be used for this purpose.

Digital optocoupler

The classic digital isolator is the LED/transistor opto-isolator. It can provide isolation upto 10kV or more. Higher speed couplers incorporate an active receiver circuit with a logic-level output.

Transformer isolation

Electromagnetic isolators such as small signal transformers are useful for AC signal isolation. Transformers like audio transformer have their primary and secondary sides isolated which can be used for different audio signal isolation. Another most common use is in network hardware or Ethernet section. Pulse transformers are used to isolate the external wiring with internal hardware. Even telephone lines are used transformer based signal isolators. But, as transformers are isolated by electromagnetically, it only works with AC.

Transformers are capable of analog accuracy of 12-16 bits and bandwidths up to several hundred kHz, but their maximum voltage rating rarely exceeds 10 kV, and is often much lower.

Capacitive isolation

The least popular method for isolating circuits is by using capacitors. Due to poor efficiency and dangerous failure outcomes this is no longer preferred. Capacitors block DC and allow passing a high-frequency AC signal. Due to this property, the capacitor is used as isolators in designs where DC currents of two circuits need to be blocked but still allowing high frequency data transmission.

Capacitive-coupled isolation amplifiers have lower accuracy, perhaps 12-bits maximum, lower bandwidth, and lower voltage ratings—but they are low cost.

Characteristics

·         Linearity- It is desirable that the relation between input and output signal is linear. Modern isolation techniques make it possible to achieve linearity as low as 0.01%.

·         Isolation voltage- It is the maximum voltage on side that can be withstand by the isolation barrier between two stages. Beyond isolation voltage, the barrier breaks down.

·         Bandwidth- It is the frequency range of input signal that can be coupled at the output without significant attenuation.

·         Transfer gain- Ratio of output signal to input signal.